1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and a FAX and the like. More particularly, the present invention relates to an image forming apparatus having a color conversion function for handling a color image.
2. Description of the Related Art
Conventionally, for performing color correction in a digital image forming apparatus, there is a technology in which a pattern image formed on an image carrier is transferred to a transfer material and fixed, and the transferred image is read by a scanner, and, then, a tone correction table is selected/generated on the basis of the read image data, so that tone correction or density correction of a printer part is performed.
Japanese laid-open patent application No. 10-191061 discloses a technology in which the tone correction table is corrected in consideration of variation of spectral characteristics of the scanner so that density values of CMYK and gray balance are corrected.
In a scanner used in an image forming apparatus like a color copying machine, due to change with time in spectral characteristics of an RGB filter in a CCD (Charge Coupled Device) constituting the scanner or due to variations of each machine, even if the same color patch pattern is read, a value read by each scanner may vary from unit to unit.
FIG. 1 is a figure for explaining non-uniformity (variation) of spectral transmission characteristics of a B (Blue) filter in a CCD. In FIG. 1, a) indicates a spectral transmission factor of a blue (B) filter 1 in a CCD, b) indicates a spectral transmission factor of a blue (B) filter 2 in the CCD, c) indicates a spectral reflection factor of yellow (Y) patch, and d) indicates a spectral reflection factor of black (K) patch. The horizontal axis indicates a wavelength, while the vertical axis indicates a spectral transmission factor or a spectral reflection factor of the CCD.
In this figure, it is assumed that the spectral transmission factors a) and b) have been shifted by a rate indicated at h) respectively, but the same consideration is applicable also to a case where the assumption as described above is not made.
Namely, comparing the light transmitted through the blue filter 1 in a) to the light transmitted through the blue filter 2 in b) under the spectral reflection factor d) of black patch, a quantity of light having transmitted through the blue filter 1 is larger by a quantity of light having transmitted through a region e), but is smaller by a quantity of light having transmitted through regions f) and g).
As shown in FIG. 1, since the spectral characteristics of the blue filter 1 in a) and the blue filter 2 in b) have been shifted by a rate in h) respectively, in a case of the light having transmitted through the blue filter 1 in a), the quantity of light having transmitted through the region e) is equal to the quantity of light blocked by the regions f) and g), and for this reason a difference for a blue signal between a) and b) is small as far as the black patch is concerned.
To strictly examine the different above, it is necessary to take into considerations the spectral characteristics of the light source and dependency of sensitivity of a CCD on wavelength, but when calibrating shading of a scanner, by using an achromatic-colored reflection plate with low dependency of a spectral reflection factor for instance in gray on wavelength in a visible light area, the difference between the filter 1 and the filter 2 is calibrated.
However, in a case of yellow (Y) patch, the difference between filters in a) and b) appears as a difference of light having transmitted through or having been blocked by the region g), and the difference is clearly larger than that in a case of black patch. Also the difference can not be calibrated even by a shading calibration using an achromatic-colored reflection plate.
The variations in spectral transmission factors among filters in a CCD can be calibrated in a case of achromatic colors like white or gray by means of shading calibration so that the RGB data become uniform, but in a case of a document with a spectral characteristic dependent on wavelength, the calibration can not be executed appropriately, and sometimes values for R, G, and B may vary unit by unit.
Due to the effect of the variation of the spectral characteristics, in a case where some machines reads the same patch having spectral reflection characteristics similar to the spectral transmission characteristics of the CCD, even if the same RGB output is set when performing shading by using an achromatic color, the outputs are different machine by machine.
The difference becomes a cause of disabling faithful color reproduction in a color calibration process for obtaining record values of cyan, magenta, yellow and black optimal for reproducing the color from RGB values of a document read by the scanner.
It is effective for correcting gray balance of achromatic color to correct the tone correction table as is disclosed in the Japanese laid-open patent application No. 10-191061 in the conventional technology for calibrating variation of the spectral characteristics of the scanner. However, there is a problem in that this technology is not applicable for reproducing chromatic color faithfully.
That is, in the invention disclosed in the Japanese laid-open patent application No. 10-191061, patch data is read by the scanner, and characteristic values of a printer table which is the tone correction table are adjusted on the basis of experiment and the like such that the density of the output of the printer after performing ACC becomes constant even when the machine is changed, so that good tone can be obtained. Therefore, since any calibration of variation of the spectral characteristics of the CCD in the scanner is not performed in the image processing part in the Japanese laid-open patent application No. 10-191061, this variation is not corrected in the input stage of the printer part, but, corrected by the tone correction table in the printer. In addition, since the correction is tone correction, it is effective for gray balance of achromatic color. However, there is a problem in that it is not enough for performing faithful color reproduction for chromatic color.